1. Field of the Invention
The present invention pertains to a microwave phase shifter which is capable, in response to a control signal, of generating a phase shift that is either substantially equal to zero or substantially equal to .pi..
2. Description of the Prior Art
The use of 0/.pi. phase shifters is known, for example in devices for modulation by 0/.pi. phase encoding or in the quantified electronic phase shifters used for each module of an electronic scanning radar antenna.
Several sorts of devices enabling the making of 0/.pi. phase shifters, either for a phase modulator or as an elementary bit in an electronic phase shifter for scanning antenna, has been proposed to date. Among these are:
Phase shifters using a PIN diode placed at the end of a normally open transmission line. The PIN diode has the role of short-circuiting this line, in thus creating a phase shift variation equal to .pi.. This phase shifter necessarily uses a circulator at the end of the line, and a circulator such as this is rather bulky. Furthermore, this type of phase shifter has limited performance characteristics, especially because of the stray capacitance of the diode which has a detrimental effect on its bandwidth.
Phase shifters using two PIN diodes placed at the end of the output arms of a 3 dB hybrid coupler. This hybrid coupler itself is also too bulky, so that this device has the same drawbacks as the previous one. PA1 Interference phase shifters formed by the cascade mounting of cells with low phase shift, using pairs of PIN diodes to load a transmission line. These are devices that are alternately switched over from a type of high-pass filter to a type of low-pass filter, by modification of the state of the diodes. This type of phase shifter may be very bulky if it is desired that it should work in wideband mode, because its bandwidth can be increased only by correlatively increasing the number of cells. PA1 Line switching phase shifters wherein two pairs of PIN diodes placed in T junctions enable the wave to go into either channel of the system, the difference in length between channels being equal to a half wavelength at the working frequency. This type of phase shifter too has the drawback of being bulky and of having a restricted bandwidth. PA1 Inversion phase shifters as described in the document FR--A--2.379.196 wherein a transmission line, of the microstrip type for example, is coupled to a slot line by means of two PIN diodes with opposite states of operation. Depending on the state of the diodes, there follows an excitation of the slot line with phases having a difference intrinsically equal to .pi.. This device has the drawback of being restricted, in terms of value of the bandwidth, to about 30% of the carrier frequency. Moreover, it practically cannot be made in integrated circuit form and is finally quite bulky.